The present invention disclosed herein relates to a balun device, and more particularly, to a marchand balun device having broadband properties.
A balun device is typically used for designing a balanced mixer, a 2-multiplier, a push-pull amplifier. Since the balanced mixer has an excellent ability for harmonic rejections and has a broad dynamic range, it is used for various kinds of Radio Frequency (RF) systems. The balanced mixer needs two signals having the same amplitude but respectively different phases. Those signals are generated by the balun device.
A typical balun device has been mainly designed using a passive device. A representative balun device includes a Lange coupler, a Rat race coupler, and a Directional coupler. Frequency bands of these couplers are known as about 10% to about 15% of a start frequency.
In order to realize a broadband balun device, a Marchand balun is being developed which consists of two coupled lines of ¼ wavelength. The loads of the marchand balun device are at the middle portion of a coupled line of which two ends are grounded. A frequency band of the marchand balun device is known as up to a frequency (i.e., 1 octave) that is two times the maximum start frequency.
If a frequency band of a balun device is broad, a frequency band of a balanced mixer using the balun device is increased also. The balanced mixer for supporting a broadband may be extensively used in a system of various frequency bands. Accordingly, a market value of the balanced mixer may become significant.
The papers related to increasing a frequency band of a balun device are published by Chin-Shen Lin et. al., titled “Analysis of Multiconductor Coupled-Line Marchand Baluns for Miniature MMIC design.” in IEEE Tans. Microwave Theory Tech., Vol. 55, no. 6, pp. 1190-1199, June 2007 and by Kenjiro Nishikawa et. al., titled “Compact and Broad-Band Three-Dimensional MMIC Balun,” in IEEE Trans. Microwave Theory Tech., vol. 47, no. 1, pp. 96-98, January 1999.
Lin connects 7 coupled lines horizontally in order to increase a coupling coefficient. As a result, a broadband balun device is realized with about 30 GHz to about 60 GHz (two times a frequency band). This balun device has an output phase imbalance of about 180°±15° and an amplitude imbalance of about ±1.5 dB. Nishikawa disposes two coupled lines vertically and uses inductor compensation to obtain an output phase imbalance of about 180°±10° and an amplitude imbalance of ±1.0 dB within a range of about 8.5 GHz to about 30 GHz (3.5 times a frequency band).
The published patent, U.S. Pat. No. 6,150,897, titled “Balun Circuit with Cancellation Element in Each Coupled Line” suggests a device for compensating a balun device for property deterioration by inserting an inductor or a capacitor in a middle portion of an output unit and a coupled line of a typical marchand balun device. This patent uses a lumped element in order to compensate for property deterioration of a balun device. However, this patent does not increase a coupling coefficient of the coupled line.